1. Field
The present invention relates generally to wireless communications systems and more specifically to equalizer timing recovery techniques in a wireless communications systems.
2. Background
In a wireless communications environment, reliable communication can be adversely affected by a number of channel impairments. Typical sources that degrade the received signal are noise, co-channel interference, intersymbol interference (“ISI”) and multipath interference. Multipath interference is often a very disruptive type of distortion to reliable wireless communication in that it produces fading.
In a coherent communications system, such as code division multiple access (“CDMA”), a radio receiver, in general, should be in constant synchronization with the radio transmitter in order to maintain a constant connection with the transmitter. If the receiver loses synchronization, a loss of “lock” with the transmitter occurs. This situation often leads to a loss of service for the user of the wireless service.
In a cellular phone environment, this can lead to dropped calls for the phone user or, in wireless data environment, to a slow, unreliable network connection for the user. Poor reliability can lead to aggravated users, who may seek to discontinue service with a provider if the situation persists. If poor reliability occurs often enough, a wireless service provider may lose market share and profit.
To combat the ill effects that a channel may cause, a well designed receiver may use a Rake processor, a fractionally spaced equalizer, or both. A Rake processor comprises one or more “fingers,” each of which tracks an individual multipath ray so that the multipath rays from each finger can be added together constructively. Rake receivers are known in the art to perform well in low carrier-to-interference (“C/I”) environments.
A fractionally spaced equalizer (“FSE”) is a finite-length tapped delay line filter whose coefficients are spaced with a time separation less than or equal to the reciprocal bandwidth of the signal. If the reciprocal bandwidth is T seconds, then as an example, the FSE tapped delay line could be spaced at T/2 or 3T/4 seconds. The filter reduces the effects of ISI from multipath interference and pulse-filtering.
In order for a FSE to properly mitigate interference, the filter must span the channel's entire impulse response. The filter's delay offset must be adjusted to ensure that this indeed is the case. Additionally, since the channel's impulse response may change with time, the FSE's delay must be continuously changed so that it is always spanning the channel's impulse response.
Known methods for determining and adjusting the delay offset for a FSE, such as using timing information of the strongest Rake finger as delay offset for the equalizer, have been found to be unreliable. There is, therefore, a need in the art for an effective method for determining and adjusting the delay offset for a fractionally spaced equalizer.